To provide energy and reducing power for the rest of the cell, the ATP and NADPH are instead used within the chloroplast stroma to produce a simple three-carbon sugar that can be exported to the cytosol by specific carrier proteins in the chloroplast inner membrane. This production of sugar from CO 2 and water, which occurs during stage 2 of photosynthesis, is called carbon fixation.

In the central reaction of photosynthetic carbon fixation, CO 2 from the atmosphere is attached to a five-carbon sugar derivative, ribulose 1,5-bisphosphate, to yield two molecules of the three-carbon compound 3-phosphoglycerate. This carbon-fixing reaction is catalyzed in the chloroplast stroma by a large enzyme called ribulose bisphosphate carboxylase or Rubisco.

Rubisco works much more slowly than most other enzymes: it processes about three molecules of substrate per second― compared with 1,000 molecules per second for a typical enzyme. The enzyme generally represents more than 50% of the total chloroplast protein, and it is widely claimed to be the most abundant protein on Earth. The energy and reducing power needed to regenerate ribulose 1,5-bisphosphate come from the ATP and NADPH produced by the photosynthetic light reactions.

The elaborate series of reactions in which CO 2 combines with ribulose 1,5-bisphosphate to produce a simple three-carbon sugar—a portion of which is used to regenerate the ribulose 1,5-bisphosphate that’s consumed—forms a cycle, called the carbon-fixation cycle, or the Calvin cycle. For every three molecules of CO 2 that enter the cycle, one molecule of glyceraldehyde 3-phosphate is ultimately produced, at the expense of nine molecules of ATP and six molecules of NADPH, which are consumed in the process. Glyceraldehyde 3-phosphate, the three-carbon sugar that is the final product of the cycle, provides the starting material for the synthesis of the many other sugars and other organic molecules that the plant needs.